Load balanced planar bearing assembly especially for a cryogenic probe station

ABSTRACT

ATE is mounted directly to a support surrounding and removably receiving probe cards of differing thickness. An electrically heated chuck having a variable heat transfer coefficient heat exchanging face confronting a liquid nitrogen supplied plenum exhibits a temperature that varies with electrical heater load, rate of liquid nitrogen supply, and heat transfer coefficient of the heat exchanging face. The rate of liquid nitrogen supply is varied as a function of whether or not the electrical heater load falls outside predetermined upper and lower bounds to establish and maintain set point temperature over a range of set point temperatures including liquid nitrogen temperature and ambient temperature. The chuck is mounted for X,Y movement on load-balanced air/vacuum bearing, and is mounted for Z movement to accommodate different thickness probe cards. A load-lock having a pivoting cassette receiving tray that pivots with a pivoting transfer arm is mounted for Z movement with the chuck. A vacuum pump selectively lowers the temperature at which the supplied liquid nitrogen boils. A positive pressure of N 2  gas is maintained to prevent moisture condensation in the load-lock and on the operative surfaces of the chuck.

This application is a division of application Ser. No. 07/431,572, filedNov. 3, 1989.

FIELD OF THE INVENTION

This invention is directed to the field of semiconductor testing, andmore particularly, to a new and improved cryogenic probe station.

BACKGROUND OF THE INVENTION

A plurality of integrated circuit devices are manufactured in well-knownmanner in and on a single wafer of semiconductor or other material.Prior to slicing the wafer to free each of the integrated circuitdevices for encapsulation, predetermined test sequences are run on eachon-wafer integrated circuit device to determine if each device isoperating in its intended manner. A probe card, which has a plurality ofdepending electrode fingers configured to conform to the particulargeometry of the integrated circuit devices manufactured on the wafer, isconnected to automatic testing equipment (ATE) which runs thepredetermined test sequences. The wafer and probecard are movedrelatively to each other until all of the integrated circuit devices onthe substrate have been tested by the ATE.

Certain classes of integrated circuit devices are intended for operationat temperatures other than ambient temperature, and cryogenic and otherprobe stations are known that are operative to provide testing of suchintegrated circuit devices at the cryogenic and other temperatures atwhich these integrated circuit devices are intended to operate. In onesuch heretofore known cryogenic test station, an enclosure is providedhaving a top opening and a removable optical window. A spring-loadedslide mechanism is provided in the enclosure proximate the opening forreceiving the probecard. A micrometer adjustment head is provided thatcooperates with the spring-loaded slide for accommodating differingthickness probecards. A chuck is provided that is movable subjacent theprobecard for receiving the semiconductor wafer having the integratedcircuit devices thereon to be tested. The chuck is provided withopenings therein for receiving resistive heating elements, and isprovided with a plenum for allowing liquid nitrogen to boil therein. Theplenum is connected to a source of liquid nitrogen via an input flowconduit, and is connected to ambient via an output flow conduit. Threedifferent exhausts of varying orifice size are provided in the outputflow conduit for selecting three different liquid nitrogen flow ratesand corresponding high, medium, and low-temperature regimes. Atemperature sensor is provided for sensing the chuck temperature and atemperature controller responsive to the temperature of the chuck isprovided for controlling the heat supplied by the resistive elements.The three different exhausts provide gross temperature control in theselected one of the high, medium, and low-temperature regimes, and theresistive elements provide fine temperature control to a predeterminedtemperature stability within any given regime of gross temperaturecontrol. A θ actuator is provided for rotating the chuck, a Z-actuatoris provided for moving the chuck up and down, and an X,Y table mountedon ball-bushings is provided for stepping the chuck in the plane of thechuck. A single-wafer load-lock is provided for inserting a wafer to betested into and for removing a wafer after testing out of the cryogenictest station. The load-lock is provided with a pivoting top coverthrough which the wafer is inserted and removed that opens to ambient,and a sliding door is provided between the load-lock and the chuck ofthe cryogenic station. A single-wafer vacuum-pickup arm is provided forlinearly moving the substrates between the top surface of the chuck andthe load-lock. The load-lock and the enclosure of the station areprovided with a positive pressure nitrogen environment for preventingmoisture condensation on the cold test surfaces of the cryogenic testingstation.

SUMMARY OF THE INVENTION

The objects of the present invention represent several improvements overthe heretofore known cryogenic probe station.

One object that is disclosed by the present invention is an improvedapparatus for receiving the probecard and for supporting ATE to whichthe probe card is connected. In accord with this object, the probecardis received in a generally flush relation to the open top of the teststation, thereby enabling an ease of card placement and removal, and ismounted to structural members of the test station. ATE placed inproximity to the probecard is supported by the structural members of thetest station, thereby enabling an ease of ATE set-up and take-down.

Another object that is disclosed by the present invention is an improvedapparatus having members extending from the chuck into the plenum of thechuck for providing different conductive heat transport rates betweenthe liquid nitrogen that is flowed through the plenum and the chuck. Inaccord with this object, conductive heat transport off the chuck isselectively available in different degrees, thereby enabling bettercontrol of the temperature of the chuck.

Another object that is disclosed by the present invention is an improvedapparatus responsive to the resistance heaters load and connected to avariable orifice positioned in the input flow conduit for controllingthe size of the variable orifice, and therewith the rate of liquidnitrogen flow, in dependence on whether or not the load is withinpredetermined bounds. In accord with this object, the predeterminedbounds for maximum stability of set point temperature are determinedsuch that the deadband defined between the bounds is enough to maintainany set point temperature with the resistance heaters load alone beingvaried between the bounds, thereby minimizing the liquid nitrogen flowrate for each set point temperature.

A further object that is disclosed by the present invention is animproved apparatus allowing the chuck to move on an air bearing in X andY to any X, Y position, and for holding the chuck absolutely stable in Zat that X, Y, position, thereby enabling jitter-free testing of theintegrated circuit devices borne by the wafer. In accord with thisobject, the chuck is mounted for movement with a load-balancedair/vacuum bearing that itself rides on a single, precision-finishedsurface plate, and in such a way that no other member but the surfaceplate is required to mechanically constrain the air/vacuum bearing.

Another object that is disclosed by the present invention is an improvedapparatus for adjusting the surface plate in Z in order to accommodatedifferent thickness probecards. In accord with this object, a manuallyand/or an automatically adjustable lifting and lowering mechanism iscoupled between the surface plate and structural members of thecryogenic test station.

A further object that is disclosed by the present invention is animproved apparatus for achieving a lower minimum set-point temperaturethan has been available from the hithertoknown cryogenic probe station.In accordance with this object, a vacuum pump is selectively couplableto the fluid flow output conduit the negative pressure of which lowersthe boiling point and thereby the temperature at which the liquidnitrogen supplied to the plenum boils.

Another object that is disclosed by the present invention is an improvedapparatus for holding plural wafers all at once in the load-lock and fortransferring any selected wafer singly to the chuck for testing and backto the load-lock after the integrated circuits thereon have been tested.In accord with this object, a cassette-load of wafers is received in apivotable carrier, and a pivoting transfer arm cooperates with thepivotable carrier to transfer wafers between the load-lock and thechuck.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and features of the present invention willbecome apparent as the invention becomes better understood by referringto the following detailed description of the preferred embodimentsthereof, and to the drawings, wherein:

FIG. 1 is a partially pictorial plan view of the improved cryogenicprobe station of the present invention;

FIG. 2 is a partially pictorial side elevation of the improved cryogenicprobe station of the present invention;

FIG. 3 is a sectional view through a diameter of the chuck of theimproved cryogenic probe station of the present invention;

FIG. 4 illustrates in FIGS. 4A, 4B, and 4C thereof sectional views ofalternative embodiments of the chuck of the improved cryogenic probestation of the present invention;

FIG. 5 is perspective view of a further embodiment of a chuck memberconstructed in accordance with the improved cryogenic probe station ofthe present invention;

FIG. 6 illustrates graphs in FIGS. 6A, 6B, and 6C thereof useful inexplaining temperature control in accordance with the improved cryogenicprobe station of the present invention; and

FIGS. 7A and 7B are bottom plan view of an air bearing constructed inaccordance with the improved cryogenic probe station of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, and primarily to FIG. 2, generallydesignated at 10 is the improved cryogenic probe station of the presentinvention. The station 10 includes structural members defining bottomframe 12, four upstanding side columns 14 joined to the bottom frame 12,and defining a top plate 16 having an opening generally designated 18provided thereinthrough. A tooling ring 20 having an outwardly radiallyextending flange 22 and an inwardly radially extending flange 24 isrotatably mounted in the opening 18 provided therefor in the top plate16 of the station 10, with the outwardly directed flange 22 in seatingrelation to the confronting wall defining the opening 18. A toolingplate 26 having a central opening generally designated 28 providedthereinthrough is mounted in seating relation to the inwardly directedflange 24 of the tooling ring 20. An optical window 30 is removablymounted in seating relation in the opening 28 of the tooling plate 26.

A probecard 32 having a plurality of depending electrode fingers 34 isremovably mounted to the inside surface of the tooling plate 26 as bythreaded fasteners, not shown. Automatic testing equipment, not shown,is connected to the electrodes 34 via flexible electrical connectors 36that are fed through openings provided therefor in the tooling plate 26.The ATE may be placed on the top of the station 10, on the tooling plate26, and on the top plate 16, and its weight is transferred to andsupported by the structural members of the frame. Different thicknessprobecards 32 may be directly mounted to the tooling plate 26.

An X, Y, Z, and θ movable chuck generally designated 40 is illustratedin FIG. 2 in its test position subjacent the probes 34 of the probecard32. In its test position, an operator, schematically illustrated at 42,is able to view probe card finger alignment and the rest via optics 44through the window 30. The optics 44 may be a microscope mechanicallyarticulated to the cryogenic probe station 10 in any suitable manner.

Referring now briefly to FIG. 3, the chuck 40 includes a metallic disc42 having a generally planer semiconductor wafer receiving facegenerally designated 44, and an opposing heat exchanging face generallydesignated 46. In the embodiment illustrated in FIG. 3, the heatexchanging face is constituted as a plurality of concentric rings 48,50, 52 the proximate ends of which all terminate in a common plane andthe wall members of which have a cross-section that tapers to theirends. The rings 48, 50, 52 are of the same material as and are machinedinto the chuck disc 42. The rings 48, 50, 52 may also be fashioned froma material different from the material of the chuck disc 42 wholly orpartly along their elongation. If the disc 42 is copper, the free endsof the rings may have stainless steel tips 54, 56, 58. The differentcross-sections with length, and/or the different materials along theirlengths, provide different coefficients of conductive heat transport.

The disc 42 of the chuck 40 is seated on and fastened to radially spacedand upstanding inner and outer annular flanges 60, 62 of chuck cupmember generally designated 64. The inner and outer flanges 60, 62 andtheir included wall, together with the heat exchanging face 46 of thechuck 42, provide an annular plenum generally designated 66 into whichliquid nitrogen is flowed and in which it is allowed to boil. An inputfluid flow conduit port generally designated 68 is provided through thecup 64 that is in fluid communication with the plenum 66. A shoulder 70is provided about the circumference of the chuck disc 42, and a shoulder72 is provided about the lower circumference of the chuck cup 64. Anannulus 74 having a "L"-shaped cross-section is seated in and fastenedto the shoulders 70, 72 of the chuck disc 42 and chuck cup 64. Theinside walls of the annulus 74 together with the confronting outsidewalls of the chuck cup 64 define an annular exhaust plenum generallydesignated 77 that extends around the periphery of the chuck 40. Aplurality of liquid nitrogen exhaust holes generally designated 78 areprovided radially through the outermost one of the concentric rings ofthe heat exchanging face 46 of the chuck disc 42 that provide fluidcommunication between the plenum 66 and the plenum 77. Typically, aboutthirty such exhaust holes are provided. An exhaust fluid flow conduitport generally designated 80 is provided through the annulus 74 in fluidcommunication with the plenum 77.

The plenum 66 may be filled with steel wool schmatically illustrated at81 to prevent boiling splatter from directly impacting the chuckunderside to improve temperature stability at comparatively highertemperatures, and to prevent splatter from being blown directly into theexhaust holes 78 to conserve the amount of liquid nitrogen usage. Ascreen, not shown, is provided in the plenum 66 across the ends of therings 48, 50, 52 to further control boiling. A thermal jacket, notshown, is provided peripherally around the side wall of the chuck 40. Aplurality of apertures dimensioned to receive electrical resistanceheaters generally designated 82, 84, and 86 are provided through thechuck disc 42. The apertures may be parallel to each other, asillustrated, or radially provided, not shown, through the chuck disc 42.The underside of the chuck disc is provided with four etched radialgrooves, not shown, to provide fluid flow paths between the remotesurface of the heat exchanging face 46 and the peripheral liquidnitrogen exhaust holes 78.

As shown in FIGS. 2 and 3, the chuck cup member 64 is mounted to apedestal 88 that is mounted for movement in X, Y, Z, and θ with chuckbase member 90. The pedestal 88 is journaled in a bearing and driven bya θ actuator, not shown, to provide θ rotation of the chuck over apredetermined angular range of rotation, and is operatively coupled to aZ acuator, not shown, to provide either up or down chuck motion to movethe chuck into contact with the probes 34 for testing or away from theprobe contacts 34 after testing of individual circuit devices.

Referring now to FIG. 4A, an alternative embodiment of the chuck discmember is generally designated at 92. The heat exchanging face thereofis constituted as an annulus that symmetrically steps in three phasesfrom its distal end to its remote end and in such a way as to exhibit aconstant radial cross-section in each of the phases thereof. Again, theheat exchanger surface may be constructed of the same or differentmaterial from that of the chuck either wholly or in part along itslength.

Referring now to FIG. 4B, another embodiment of the chuck disc memberaccording to the present invention is generally designated at 94. Theheat exchanging face of the FIG. 4B embodiment is constituted as aplurality of concentric rings, with radially outer rings being longerthan radially inner rings, and with each ring having a "V"-shaped radialsection. Like in the embodiments of FIGS. 3 and 4A, the material of theconcentric rings may be the same or different from the material of thechuck disc member either wholly or partly along their lengths.

Referring now to FIG. 4C, generally designated at 96 is a furtherembodiment of a heat exchanging face of the chuck disc member inaccordance with the present invention. Like for the FIG. 4B embodiment,the heat exchanging face illustrated in the FIG. 4C embodiment isconstituted as a plurality of annuli, with radially outer annuli beinglonger than radially inner annuli, but which exhibit a constant radialcross-section along their lengths. The heat exchanging face 96 may beconstituted of materials that are the same or different from thematerial of the chuck disc member either wholly or partly along itslength.

Referring now to FIG. 5, generally designated at 98 is yet anotherembodiment of a heat exchanging face of the chuck disc member inaccordance with the present invention. In this embodiment, the heatexchanging face is constituted as an annulus that circumference stepseither up or down in three phases with change in angle about a givenradius. Four flat peaks and four flat valleys symmetricallycircumference arranged are illustrated, although any number of peaks andvalleys may be employed. The heat exchanging face of the FIG. 4Cembodiment exhibits a uniform radial cross-section at any given angle,although it will appreciated that a varying cross-section such as atapering cross-section can be employed as well. Again, the material ofthe heat exchanging face may be the same or different from that of thechuck disc member either wholly or partly along its length.

Other heat exchanging face embodiments are contemplated, and are onlylimited to any such embodiment that has a preselected parameter selectedsuch that the coefficient of heat transport rate provided by the heatexchanging face varies along the dimension of its extension whether thatparameter is associated with material, geometry, or otherwise.

Returning now to FIGS. 1 and 3, the fluid flow input port 68 isconnected to a source 100 of pressurized liquid nitrogen via conduits102, 104, 106 respectively interconnected via flex joints generallydesignated 108, 110, and 112. An on/off valve 114 is providedimmediately downstream of the pressurized source of liquid nitrogen 100and a variable orifice 116 is provided immediately upstream of the valve114. The chuck 40 is connected to atmosphere along an exhaust outputfluid flow path having conduits 118, 120, and 122 respectivelyinterconnected by the flex joints 108, 110, and 112. Downstream of theflex joint 112 and exhaust heater 124 is provided for expanding theliquid nitrogen in the output fluid flow path. A three-way valve 126 ispositioned downstream of the exhaust heater 124. The three-way valve 126has one state that vents the exhaust to atmosphere, as marked, andanother state that connects the fluid flow output conduit to atmospherevia a vacuum pump 128 relieved by a pressure valve 130. Liquid nitrogensupplied into the input port 68 flows into the plenum 66 of the chuck ata rate that is determined by the size of the variable orifice 116. Independance on the rate, it fills the plenum to different levels, and isin contact with a varying surface area of the heat exchanging face 46.As it boils, it is exhausted through the exhaust holes 78, exits theexhaust port 80 via the plenum 77, and is either discharged toatmosphere via the valve 126 or is coupled to the vacuum pump 128. Thepump 128, selected for maximum low temperature operation, reduces theboiling point of the liquid nitrogen, thereby enabling lowered set pointtemperatures to be achieved.

A temperature sensor 130 is embedded in the chuck 40 and coupled to atemperature controller 132. The temperature controller 132 is responsiveto a set point temperature as marked, and to the temperature of thechuck provided by the sensor signal, to control driver circuitry 134connected to electrical resistance heaters 136 embedded in the apertures82, 84, 86 provided therefor in the chuck disc 42. The driver 134 variesthe power level to the heaters, and is preferably a linear non-switchingD.C. amplifier that minimizes electrical noise at the chuck, which couldaffect sensitive measurements. A.C. power and on-off controllers orother driver circuitry could be used if electrical noise is not aproblem. A motor controller 138 is responsive to the driver outputsignal to control a motor 140 that varies the orifice size of thevariable orifice valve 116. The motor controller 138 has adjustable,preselected upper and lower bounds respectively marked "T₁, T₁ ".

The temperature set point may be selected to be any temperature within arange of temperatures from 75° K. to as high as 425° K. with atemperature stability of +/-0.5° K. The temperature of the chuck isestablished at any set point within the range of temperatures andmaintained there in dependance on the electrical heater load, the liquidnitrogen flow rate, and the operative heat transfer coefficient beingexhibited by the heat exchanging face of the chuck disc member.

The preselected lower temperature bound "T₁ " of the motor controller isselected such that there is always delivered to the electricalresistance heaters that minimum power that allows the heat producedthereby to insulate the temperature of the chuck against temperatureinstability associated with uncontrolled and undesired boiling, asoccurs, for example, in the flow paths, and against other liquidnitrogen introduced instabilities. Where maximum cold temperatureoperation is important, or where temperature stability is not important,the lower bound may be set arbitrarily low.

The preselected upper bound "T_(u) " of the motor controller is selectedsuch that the deadband defined between it and the lower bound issufficient to provide intended temperature stability at a giventemperature set point by only varying the load of the electricalresistance heaters while maintaining a constant value for the size ofthe variable orifice of the variable orifice valve 116. In this manner,the usage of liquid nitrogen is kept at the minimum required toestablish any set point temperature for a given temperature stability.Different deadbands may be employed for operation in differenttemperature regimes.

Referring now to FIG. 6, generally designated at 142 in FIG. 6A is agraph of chuck temperature plotted against time, generally designated at144 in FIG. 6B is a graph of percentage of liquid nitrogen flow rateplotted against time, and generally designated at 146 in FIG. 6C is agraph of percentage of heater power plotted against time. In a firstinstance, the graphs 142, 144, 146 illustrate operation for an exemplarychange in temperature set point from 77° K. to 150° K. At the 77° K.temperature set point, the vacuum pump is turned off, the heater poweris turned off, and a fully dilated value is supplied to the variableorifice valve. The maximum rate of liquid nitrogen is then supplied bythe liquid nitrogen source, the liquid nitrogen substantially fills thechuck plenum, and thereby the wafer testing surface thereof is tied to77° K. by conductive heat transport. When the temperature controllerreceives the change to the 150° K. set point temperature, it turns theheater power fully "on" so as to deliver maximum heating power to thechuck via the linear amplifier driver connected to the electricalresistance heaters. In response to the heater power being above theupper temperature bound, the motor controller linearly ramps thevariable orifice valve to a more throttled condition via the motorconnected thereto. As the heating increases and the cooling ratedecreases, the temperature of the chuck approaches set pointtemperature. With less flow of liquid nitrogen, less heater power isrequired. The motor controller continues to linearly throttle the sizeof the variable orifice valve until the electrical resistance heater'sload falls within the deadband of the motor controller as illustrated byline 148, whereupon the liquid nitrogen flow rate is stabilized byturning the motor controlling the variable orifice valve off. Asillustrated by a line 150, the percentage of heater power correspondingto the load being sensed once again exceeds the upper bound, and themotor is again actuated to throttle the variable orifice further. Withlessened liquid nitrogen flow rate, lesser heater power is required, andas illustrated by a line 152, the percentage of heater powercorresponding to the electrical resistance heater load falls into thedeadband of the motor controller, whereupon the motor is again turnedoff again giving another constant liquid nitrogen flow rate. Withstabilized lessened flow rate, the heater load is reduced, and asillustrated by a line 154, as soon as the percentage of heater powercorresponding to the electrical resistance heater load drops below thelower bound of the deadband of the motor controller, the motor linearlydilates the variable orifice valve, producing a corresponding increasein the percentage of liquid nitrogen flow rate. This process continuesuntil the percentage of heater power crosses back into the deadband ofthe motor controller as illustrated by a line 156, whereupon, the liquidnitrogen flow rate is again maintained at a uniform value. Asillustrated by a line 158, the set point temperature is then maintainedsolely by varying the percentage of electrical resistance heater powerat the 150° K. set point temperature.

In a second instance, the graphs 142, 144, 146 illustrate operation foran exemplary change in set point temperature from 150° K. to 100° K. Asalso illustrated by the line 158, the heater power is turned off, andthe electrical resistance heater load falls outside the lowertemperature bound of the motor controller upon receipt of this change inset point temperature. The flow rate of the liquid nitrogen remainsconstant until such time as the percentage of heater power crosses thelower temperature bound as illustrated by a line 160, whereupon, themotor controller ramps open the orifice of the variable orifice valve tocause a larger percentage of liquid nitrogen to be supplied to thechuck. As the flow rate of liquid nitrogen is increased, the temperatureof the chuck approaches the lowered set point temperature, and thetemperature controller again turns the electrical resistance heaters on.The electrical resistance heaters load crosses the lower bound as shownby a line 162, whereupon, the liquid nitrogen flow rate is stabilized.As illustrated by a line 164, if the percentage of heater power exceedsthe upper bound, the percentage flow rate of liquid nitrogen is reduced.The temperature of the chuck accordingly rises, requiring less heaterpower, until the electrical resistance heater load is pulled back downagain into the deadband of the motor controller. As illustrated by aline 166, operation at the higher set point temperature is thenmaintained by only varying the load to the electrical resistanceheaters.

Referring now again to FIGS. 1 and 2, the base 90 of the chuck 40 has anair bearing illustrated in dashed outline 170 on which it rides over theconfronting surface of a surface plate generally designated 172, and hasan air bearing generally designated 174 on a side surface thereof onwhich it rides along the confronting upstanding wall of a T-slidegenerally designated 176. The T-slide has air bearings illustrated indashed outline 178 on its undersurface on which it rides over theconfronting surface of the surface plate 172. The cross member 179 ofthe T-slide 176 has upstanding air bearings illustrated in dashedoutline 180 on which it rides along the confronting upstanding surfaceof the surface plate 172. An X actuator and position sensor, not shown,are provided for sliding the chuck against the confronting surface ofthe T-slide to assume any intended surface plate X coordinate, and a Yactuator and position sensor, not shown, are coupled to the T-slide forcausing the T-slide, and therewith the chuck, to assume any intendedsurface plate Y coordinate.

Referring now to FIG. 7, generally designated at 180 is a preferredembodiment of any one of the air bearings 170, 174, 178 and 180. The airbearing 180 includes a central vacuum pad generally disignated 182 and afour-segment channel generally designated 184 symmetrically disposedsurrounding the central vacuum pad 182. Each of the segments includes ametered orifice generally designated 186. Vacuum imparted to the vacuumpad 182 draws the bearing 180 to its confronting surface. Meterednitrogen is supplied through the four-segment channel, and pushes thesame confronting surface away from the air bearing. The symmetricaldistribution of the four-segment channel provides uniform repulsiveforce about the four corners of the bearing, while the central vacuumpad provides uniform attractive force. The metered orifices 186, in eachof the symmetrically disposed segments, provide constant gas flow nomatter what the load distribution on the bearing plate might be. The airbearing 180 thus exhibits a gap dimension that is stable against unequaland equal load distributions about the bearing surface. Where thenatural frequency of the air bearing produces instability, the dynamicstability of the bearing can be increased by tuning each air bearingchannel to a different natural frequency by slightly differing orificesizes as shown at 188 and 188' in FIG. 7B, or differential bearinggeometry, such as groove dimensions is shown at 189 and 189' in FIG. 7B,or by varying the pressure of the gas to the metering orifices. Whilefour segments are shown, a smaller number, not less than three, or agreater number, can be employed, so long as all or most of the segmentshave their own metered supply of gas. The dynamic equilibriumestablished by the play between the negative vacuum and positive gaspressures holds the confronting surfaces together while permitting theirrelative sliding movement in such a way that the confronting surfacesare themselves held together in spaced apart relation with no otherconstraining members being required. When the gas is turned off, thevacuum sucks the confronting surfaces together, providing absolutepositional stability in the X, Y and Z direction.

Referring once again to FIGS. 1 and 2, the surface plate 172 is mountedon uprights 190 that are slidably mounted for up and down Z movement toframe bottom member 12. The height of the surface plate is adjustablemanually by micrometer adjustment posts, not shown, and/or automaticallyby a Z actuator, not shown, to that Z position that accommodates theparticular thickness of the probe card 32 being employed for a specifictest configuration.

A transfer mechanism generally designated 192 has base generallydesignated 193 thereof mounted for Z movement with the surface plate172. The transfer mechanism 192 includes an arm 194 pivotably mounted ona post 196 journaled in base member 194. The arm 194 of the transfermechanism 192 pivots between a load-lock position illustrated in solidline, and a transfer position illustrated in dashed outline. A load-lockchamber generally designated 200 is provided adjacent the front wall ofthe improved cryogenic probe station, and it includes an outer pivotingdoor illustrated in dashed line 202 and an inner pivoting doorillustrated in dashed line 204. A cassette carrier 206 that is pivotallymounted on a shaft 208 journaled for rotation in the base member 193 ofthe transfer mechanism 192 is positioned in the load-lock 200 andaccepts a cassette 210 that is able to accept a plurality ofsemiconductor wafers therein. A cam 212 is mounted for rotation with theshaft 196 of the transfer mechanism 192, and a cam follower 214 inbearing relation to the cam 212 is attached via an arm 215 to thepivoting shaft 208 of the carrier tray 206.

In operation, the arm 194 of the transfer mechanism is raised to aheight corresponding to a selected wafer position of the cassette asillustrated in dashed line in FIG. 2, and as it pivots thereto ortherefrom about its pivot 196, the carrier tray 206 follows the pivotingmotion via the cam follower 212 to align the mouth of the cassette withthe arc of the transfer arm thereby enabling contact-free waferinsertion and removal. The load-lock 200 is purged with a nitrogen gasstream at positive pressure to prevent moisture condensation. The outerdoor 202 and inner door 204 enable cassette placement and removal whiletesting is underway. As illustrated in dashed outline 212, the cryogenicprobe station is maintained at a positive nitrogen pressure to preventmoisture condensation upon the operative surfaces of the cryogenic probestation.

While liquid nitrogen is disclosed in the preferred embodiments, otherliquefied gases such as liquid helium or liquid H₂ could be employed aswell without departing from the inventive concept.

The present invention has application to testing any circuit deviceborne on any type of substrate and is not limited to the integratedcircuit devices on the semiconductor wafers as in the examplaryembodiments. Superconducting materials, ceramic based large scaleintegrated circuits and infrared sensing devices are contemplated.

Many modifications of the presently disclosed invention will be apparentto those skilled in the art without departing from the inventiveconcept.

What is claimed is:
 1. A planar bearing cooperative with the confrontingsurface of a planar race to provide two degrees of freedom movement ofthe bearing over the race and in such a way that the bearing isconstrained against movement in a third degree of freedom, comprising:amember having a substantially flat bearing surface, a central vacuum padformed in the flat bearing surface, at least four gas supply channelsformed symmetrically around the vacuum pad, and at least four meteredgas supply orifices formed in respective ones of the at least four gaschannels to provide a preselected, constant rate of gas supply that isindependent of bearing load; wherein the member has four sides andwherein each of the at least four gas supply channels is formed as atwo-legged channel positioned with corresponding legs in spaced relationto two different sides.
 2. The invention of claim 1, wherein the foursides are arranged as a rectangle and the two-legged channels areright-angled.
 3. The invention of claim 1, wherein at least some of themetered gas supply orifices are differently sized to provide dynamicstability.
 4. The invention of claim 1, wherein at least some of the gaschannels are differently sized.
 5. A planar bearing cooperative with theconfronting surface of a planar race to provide two degrees of freedommovement of the bearing over the race and in such a way that the bearingis constrained against movement in a third degree of freedom,comprising:a member having a substantially flat bearing surface, acentral vacuum pad formed in the flat bearing surface, at least threegas supply channels formed symmetrically around the vacuum pad and atleast three metered gas supply orifices formed in respective ones of theat least three gas channels to provide a preselected, constant rate ofgas supply that is independent of bearing load, at least some of the atleast three metered gas supply orifices are differently sized to providedynamic stability.
 6. A planar bearing cooperative with the confrontingsurface of a planar race to provide two degrees of freedom movement ofthe bearing over the race and in such a way that the bearing isconstrained against movement in a third degree of freedom, comprising:amember having a substantially flat bearing surface, a central vacuum padformed in the flat bearing surface, at least three gas supply channelsformed symmetrically about the vacuum pad, at least some of said atleast three gas channels are differently sized, and at least threemetered gas supply orifices formed in respective ones of the at leastthree gas channels to provide a preselected, constant rate of gas supplythat is independent of bearing load.